Add extensible notification providers + UI/API

Introduce a provider-based notification system and wire it through the API and admin UI. Added INotificationProvider + notification service implementation and providers (apprise, discord, ntfy, pushover), plus a GET /api/admin/notifications/providers endpoint to expose provider metadata. Refactored code to use provider type strings (removed enum coupling), updated masking/encryption calls, and simplified the test notification endpoint to accept backendId or type+config and call sendToBackend directly.

UI: NotificationsTab now fetches provider metadata and renders provider cards and dynamic config forms (fields driven by provider metadata). Added config field rendering, improved backend cards, and edit/delete actions.

APIs: New providers route, updated admin notification CRUD routes to validate provider types dynamically, updated test route schema. Added download-client categories POST API to fetch categories from clients and wired postImportCategory handling in download-client routes.

Other notable changes: BookDate now fetches Claude models dynamically from Anthropic's Models API; added paginated model fetch helper. Added ALLOW_WEAK_PASSWORD flag exposure to auth providers and password change logic. Doc updates and various tests added/updated. File-organization doc clarifies EPERM fix using stream-based copy.

src/app/api/setup/download-client-categories/route.ts

@@ -0,0 +1,63 @@
+/**
+ * Component: Setup Wizard Download Client Categories API
+ * Documentation: documentation/setup-wizard.md
+ */
+
+import { NextRequest, NextResponse } from 'next/server';
+import { getConfigService } from '@/lib/services/config.service';
+import { getDownloadClientManager, DownloadClientConfig } from '@/lib/services/download-client-manager.service';
+import { SUPPORTED_CLIENT_TYPES } from '@/lib/interfaces/download-client.interface';
+import { requireSetupIncomplete } from '@/lib/middleware/auth';
+import { RMABLogger } from '@/lib/utils/logger';
+
+const logger = RMABLogger.create('API.Setup.DownloadClientCategories');
+
+/**
+ * POST - Fetch categories from a download client during setup wizard
+ */
+export async function POST(request: NextRequest) {
+  return requireSetupIncomplete(request, async (req) => {
+    try {
+      const { type, name, url, username, password, disableSSLVerify } = await req.json();
+
+      if (!type || !url) {
+        return NextResponse.json(
+          { success: false, error: 'Type and URL are required' },
+          { status: 400 }
+        );
+      }
+
+      if (!SUPPORTED_CLIENT_TYPES.includes(type)) {
+        return NextResponse.json(
+          { success: false, error: `Invalid client type. Must be one of: ${SUPPORTED_CLIENT_TYPES.join(', ')}` },
+          { status: 400 }
+        );
+      }
+
+      const testConfig: DownloadClientConfig = {
+        id: 'setup-categories',
+        type,
+        name: name || type,
+        enabled: true,
+        url,
+        username: username || '',
+        password: password || '',
+        disableSSLVerify: disableSSLVerify || false,
+        remotePathMappingEnabled: false,
+      };
+
+      const configService = getConfigService();
+      const manager = getDownloadClientManager(configService);
+      const service = await manager.createClientFromConfig(testConfig);
+      const categories = await service.getCategories();
+
+      return NextResponse.json({ success: true, categories });
+    } catch (error) {
+      logger.error('Failed to fetch categories', { error: error instanceof Error ? error.message : String(error) });
+      return NextResponse.json(
+        { success: false, error: error instanceof Error ? error.message : 'Failed to fetch categories' },
+        { status: 500 }
+      );
+    }
+  });
+}